1. Field of the Invention
The invention relates to the field of interlocks and position indicators, particularly those employing optical techniques.
2. Description of the Prior Art
In the prior art, a number of situations exist in which it is desired to confirm, visually or otherwise, the engagement or placement of a subassembly or unit in a rack or mounting arrangement. Typical of such a requirement is the commercial airliner situation in which a standardized rack mounts and provides for the connection of various electronic subassemblies such as radio communication and intercom equipment, navigation equipment, and the like.
The commercial airlines, through an industry-owned organization called "ARINC" have established so-called ATR standards for unit sizes, standardized electrical connections and mounting interface hardware. The term, LRU, refers to the lowest or least (smallest) replaceable unit in such an arrangement. When these LRUs are installed in an equipment rack, or removed and replaced for any reason, it is important that there be some readily identifiable indication that the unit is indeed fully and completely in place, so that the electrical connection of the subassembly are reliably mated to corresponding fixed connections in the mounting arrangement. Such arrangements for that assurance have sometimes been called "interlocks".
An interlock, or position-indicating arrangement or device, can obviously be provided by a mechanical means and also can be purely electrical, i.e., in which an electric switch is actuated or deactuated when the unit is slid into place. Reliability and cost considerations are obviously important. Purely mechanical devices are relatively expensive and are subject to wear, maladjustment or outright malfunction. Electric switches under all circumstances including shock and vibration and other adverse environmental factors leave much to be desired from the point of view of reliability and fail-safety.
In electrical systems, there is always some inherent fire and explosion hazard in addition to the other problems aforementioned.
The manner in which the present invention deals with the disadvantages of prior art approaches to the problem of reliable electrical subassembly engagement indication will be understood as this description proceeds.